Core/sheath fibers and yarns are well known in the existing prior art. They comprise at least two fiber-forming materials, usually polymers of different types and/or of different properties. At least one of the polymers forms the core of the finished fiber, while at least one other forms the sheath. It is a goal of the prior art to produce perfectly concentric structures, which constitute superior products.
Such fibers have substantial advantages. For example, by appropriately selecting the materials of which the core and sheath are made, the overall mechanical properties of the fiber may be varied widely. One set of properties is found in the sheath, while another set of properties results from the core.
For example, flame resistance can be imparted to the core by the use of certain additives; at the same time, the sheath can be selected for its strength or load carrying properties. Similarly, complementary fibers of this type can be used to prepare filter materials. Also, contrasting properties can be provided where needed.
However, if thin fibers to be spun into yarn are desired, the previously known equipment for doing so was highly complex and awkward to use. Due to the separate supply lines necessary to bring the plurality of fiber-forming materials to each die capillary (of a plurality of die capillaries) in order to form filaments, the devices comprise a large number of complicated individual parts which are expensive to produce. Moreover, dismantling, cleaning, and servicing such devices becomes a delicate and time consuming operation.
In one previously known pack, the core polymer is introduced through tubular members which extend into the material forming the sheath. In order to accomplish this, the pack is made up of an upper distributor and a lower spinneret plate. The latter contains die capillaries having very small diameters. At the same time, the entry channels are relatively wide in order to receive the core tubes. The device is set up so that all of the tubes are as concentric as they can be to the entry channel of the die.
However, this device suffers from a number of serious disadvantages. Most importantly, it is not possible to get more than four die capillaries per square centimeter of die area. Furthermore, the delicate nature of the fine tubes presents serious problems in dismantling, cleaning, and reinstallation. A further serious problem resides in the positioning of the core tubes. Due to the fragility of these tubes, cleaning and servicing of the pack virtually precludes maintaining the core tubes precisely concentric during the life of the devices, without the necessity of extreme care and adjustment.
When the above maladjustment occurs, and core/sheath fibers having substantial differences in viscosity between the core and the sheath are being produced, a substantial proportion of the individual fibers will exhibit pronounced "kneeling" and have a tendency to stick to the spinneret plate, thus interrupting production. Kneeling occurs when two fiber-forming materials each occupy a certain proportion of the total cross section of flow and both are subjected to the same pressure conditions. This will force them into different flow behaviors resulting from the different viscosities; the lower viscosity component will flow more rapidly so that the cross section of its flow will be reduced.
After extrusion from the die capillary, the speeds of flow of the sheath and core are matched once again. Thus, the two materials again occupy the original proportion of the total cross section dictated by their respective volumes. However, due to normal inertia of the fiber, there is a delay before the matching of speeds occurs. Therefore, the low viscosity component is still moving faster than the high viscosity component after the fiber passes through the die orifice. In such a case, if the components are not precisely concentric, the fiber will kneel as a result.
The problem of precisely centering the various channels with respect to each other is a serious one. There are many factors which cause unpredictable variations, even after the devices have been manufactured. Obviously, there are the ever-present unavoidable production tolerances, both as to the location of the centers of the channels and the positions of the receiving bores for the locating pins on the elements of packs.
Moreover, even if the pack is properly set when new, it can easily become misaligned due to the necessary servicing during its life. The necessary disassembly, cleaning, reassembly, adjustment, etc. all provide opportunities for misalignment. As a result, it becomes difficult (and hence expensive) to provide and maintain devices which will produce fine fibers in core/sheath form.
If the production cost for such fibers are to remain within economically acceptable limits, extremely close tolerances simply cannot be used. Since a large number of die capillaries are necessary, allowance must be made for the substantial portion which will exhibit the variations due to tolerance and handling. Extensive spinning tests have corroborated this.
One attempted solution has been to guide the core tubes into the channel openings by suitable elements. Those which are star-shaped have frequently been used. However, it is not possible to obtain a high die capillary density and the costs and complications of such devices (particularly during cleaning and reassembly) render them unsatisfactory.
In U.S. Pat. No. 4,052,146, there is disclosed a device wherein the annular cavities (which form the sheath) are offset vertically from each other so that they can be partially "interleaved". The annular channels which form the sheath are flat and arranged around the extension of the die channels. However, the external diameters thereof limit the capillary density which can be obtained, even if the annular channels are offset in height and overlapping. Even with this arrangement, however, the density of the die capillaries achieved is still only less than three per square centimeter.
In European Application 284,784, the core tubes are replaced by lamellas which are fixed together and traverse entire rows of the inlet openings. This assists in alleviating some of the foregoing problems; however, misalignment of the core-forming and sheath-forming elements still occurs because of unavoidable manufacturing tolerances. The cylindrical side channels require greater volume than the usual spinning channels and the separate polymer feeds may result in differences in the thickness of the sheath. This, of course, will produce the kneeling effect previously described herein. Moreover, all of the known devices, because of the high precision necessary in their manufacture, are relatively expensive.